1. Field of the Invention
The present invention is related to thermoelectric materials and improvements in the figure of merit of these materials. More specifically, it relates to a p-type polarity material for mid-temperature (200-500° C.) power generation.
2. Description of the Related Art
Thermoelectric (TE) materials have been among the most compelling and challenging materials studied during the last decade. Improvements in thermoelectric performance require a better understanding of how the optimal parameters can be achieved in a given system. Two promising groups of TE materials are based on GeTe and PbTe narrow-band semiconductors. GeTe is a p-type semiconductor in which the conductivity is determined by vacancies on the Ge sites. These vacancies affect not only the electric properties via generation of two holes per vacancy, but also contribute to phonon scattering with a reduction in lattice thermal conductivity. This makes GeTe a unique matrix where doping with various elements can significantly affect multiple mechanisms responsible for the thermoelectric properties.
Doping of GeTe with Ag and Sb produces a system that is typically written as (GeTe)y{AgSbTe2)1-y, and for which the acronym “TAGS” is commonly used. For y=85%, the material is referred to as TAGS-85, and can be described by a nominal composition of Ag6.52Sb6.52Ge36.96Te50.00. Although TAGS-85 has been used in numerous important applications, it continues to attract interest because of the strong dependence of the carrier concentration and lattice thermal conductivity on the presence of Ge vacancies and because it has one of the highest ZT value of p-type thermoelectrics. Numerous studies have examined the effect of varying the Ag to Sb ratio, but these have not resulted in a substantial improvement in ZT. Because of the co-dependence of Seebeck coefficient and electrical conductivity on carrier concentration, increasing one generally results in a decrease of the other. One of the ways to uncouple these transport parameters is to increase the density of states near the Fermi level, as recently demonstrated by addition of Tl to PbTe.
Doping with rare earth atoms can, in principle, affect transport properties of thermoelectric materials via three mechanisms, by forming: (i) enhanced electron states near the Fermi level, (ii) local defects resulting in additional carrier scattering, and/or (iii) additional carrier scattering due to localized magnetic moments. Ce, Eu, and Yb rare-earth elements can form resonance electron states near the Fermi level and strongly affect electronic transport properties, particularly thermopower. This has been observed in binary compounds, e.g., in CeAl3, YbAl2, and YbAl3, and in ternary compounds, e.g. in RM2X2 with R═Ce, Eu, Vb, M=Mo, Fe, Co, Ni, Cu, and X═Si, Ge. Doping of GeTe with 3d and 4f-atoms forms a dilute magnetic semiconductor (DMS), e.g. Ge1-xMnxTe.